Several reasons:
1. Parties aren't as big tents as they used to be - "conservative Democrat" is now an oxymoron. Many of the WWC Dems have always been conservative (minus the "beam me up," Jim Traficant might be a good example of the average WWC voter's views).
2. When many WWC Democrats voted for Nixon and Reagan, liberals started accusing them of "racism" (which may or may not be true but is irrelevant). Naturally, they resent being called racist and started voting for Republicans in higher numbers, which resulted in more accusations of racism, which resulted in more voting for Republicans, etc. A feedback loop, essentially.
3. The number of WWC voters is shrinking, especially in unionized industries that favor Democrats. The WWC voters that remain were always relatively more likely to vote Republican. In addition, the WWC is an aging demographic and voting patterns become more conservative as people age.
You know what they say don't you?
Facts have a liberal bias. And everybody is racist
